1. Field of the Invention
The present invention relates to image formation, and more particularly, to an image forming apparatus and an image forming method for forming a latent image with multiple beams.
2. Description of the Related Art
With the functional advancement of an image forming apparatus, the image formation rate of the image forming apparatus per unit time, i.e., the number of prints per minute (PPM) increases. In recent years, to achieve formation of a higher-resolution image at a higher speed, an image forming apparatus which performs multi-beam exposure using a vertical cavity surface emitting laser (hereinafter, referred to as a “VCSEL”) has been proposed. Furthermore, in response to a request for resource saving, a type of image forming apparatus capable of duplex printing has been available.
Therefore, in an automatic duplex printing apparatus, with improvement in the PPM, a time interval between printing an image on the first side of a sheet and printing an image on the second side tends to be shortened. For example, in high-speed types of duplex printing apparatuses, some apparatuses perform printing images on the first and second sides within 10 seconds.
However, with the increase in speed, a distance for conveying a sheet between heat fixing of an image on the first side and printing of an image on the second side tends to be shorter, and a time to cool down the sheet after the sheet leaves a high-temperature section is reduced, so the printing sheet becomes a state susceptible to the thermal influence.
When duplex printing is performed in such a state, in the case where an 80-micrometer-thick high-quality sheet is used as a printing sheet, it has been confirmed that a magnification difference of 0.2% to 0.4% between images printed on the first and second sides corresponding to the front and back of the sheet occurs due to changes in heat and humidity.
To cope with the above described problem, there has been conventionally disclosed a method to provide a sub-scanning magnification changing function to an image forming apparatus to eliminate a magnification difference so that the image forming apparatus can reduce an image by culling sub-scanning image data or enlarge an image by adding image data (for example, Japanese Patent Application Laid-open No. 2009-83472).
However, in the method disclosed in Japanese Patent Application Laid-open No. 2009-83472, it is impossible to resolve image deterioration caused in the image enlarging process. Specifically, there is a problem that the higher the resolution of an image to be formed, such as an image having the periodicity, for example, that a 1-line line is formed every 5 lines, the more conspicuously a global image defect, such as uneven density or moiré, appears when a line is culled or added to adjust the magnification.
Furthermore, with the process to eliminate a magnification difference, it is necessary to prevent banding caused by interference between a screen ruling and a magnification ratio or the like.